According to CBS News, the pilot can be heard on LiveATC saying that there is a problem and requesting a lower altitude. There is some delay in ATC working out a lower altitude, and eventually clears the plane to only FL200. Which is acknowledged, but the plane never descends and no further communication.

This is a tragic reminder that when you have an emergency, don't hesitate to STATE that it is an emergency to get priority handling and don't hesitate to exercise PIC authority when necessary. At 25,000 the time of useful consciousness is just a few minutes.

IIRC on the TBM, the oxygen masks are under the seats. Not the most obvious location to remember when already suffering hypoxia.